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Cock sperm competition

Sperm competition is a form of post-copulatory sexual selection [1] whereby male ejaculates simultaneously physically compete to fertilize a single ovum. Physiological evidence, including testis size relative to body weight and the volume of sperm in ejaculations, suggests that humans have experienced a low-to-intermediate level of selection pressure for sperm competition in our evolutionary history. Evidence suggests that, among the great apes, relative testis size is associated with the breeding system of each primate species. The volume of sperm in ejaculates scales proportionately with testis size and, consistent with the intermediate weight of human males testis, ejaculate volume is also intermediate between primates with high and low levels of sperm competition.
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Sperm competition

Cock sperm competition
Cock sperm competition
Cock sperm competition
Cock sperm competition
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Sperm-free sex keeps hens happily faithful | New Scientist

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Human sperm competition

By Anna Gosline. Possessive cockerels use fake sex to keep their hens faithful. By merely mounting females — without bothering to waste precious sperm — cocks ensure their partners will not go looking for male competitors to fertilise them, a new study suggests. The finding may explain why males of many species — from insects to mammals — engage in seemingly meaningless sperm-free sex. While sperm was always thought of as a cheaper investment than eggs, in the past few years, researchers have begun to realise that sperm also carries a hefty biological price tag.
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Fertilization success in sperm competition is often determined by laboratory estimates of the proportion of offspring sired by the first P 1 or second P 2 male that mates. However, inferences from such data about how sexual selection acts on male traits in nature may be misleading if fertilization success depends on the biological context in which it is measured. We used the sterile male technique to examine the paternity of the same male in two mating contexts in the burying beetle, Nicrophorus vespilloides , a species where males have alternative mating strategies based on the presence or absence of resources. We found no congruence in the paternity achieved by a given male when mating under different social conditions.
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